This invention relates to conversion of linear motion into circular or rotary motion (or vice versa) and particularly to mechanisms for conversion.
Linear motion such as a leg pushing down on a pedal, or a cylinder travelling down a shaft defines a down and up cycle that is typically converted to circular motion by means of a crank, as for example, bicycle cranks and pedals, or a crankshaft in an automotive engine. The down and up cycle consists of a down or power stroke where energy is being applied and converted to circular motion, and an up or travel stroke. With a crank, the power and travel strokes are identical.
The transfer of energy itself is not linear but follows the curve of a circle. FIG. 1 represents cycles of pedal and crank motion for a bicycle with power and travel of one pedal shown in solid line A, and the other pedal in dash line B. The median line C shows power and travel motion is symmetrical for each pedal. The more horizontal the curve shown in solid line, the more efficient is the energy transfer. There are only two points in a cycle at which the vector of power is applied perpendicular to the crank and where power application is 100% efficient. Also, for each cycle, there are two points where no energy is being transferred, no matter how much force is being applied. These are the “O” power positions of FIG. 1. With bicycle pedals “O” positions are overcome by legs moving back and forth as well as up and down. So, when one pedal is fully down and the other fully up, we apply forward motion to “nudge” the pedals into position where power can be applied. With an engine crankshaft, the “O” position is overcome by flywheel inertia.